CUMULATIVE EFFECTS OF GENES 



with white grain. The white-grained plants bred true. So did some 

 of the red-grained ones — 37 out of the 63 of them, to be precise. 

 But of the remaining red-grained plants, some gave 3 : i ratios in 

 F3, some 15:1 ratios and some repeated the 63 : i ratio. Nilsson- 

 Ehle reahzed that the parents must have differed by 3 genes (which 

 we can now relate to their having six sets of chromosomes instead 

 of the normal two), the red-producing allelomorph being dominant 

 in each of them, and a single red-producing allelomorph being in 

 itself sufficient, no matter to which gene it belonged, to give a red- 

 grained plant. The genes were all like one another in their effects. 



This similarity went even deeper. The reds, though all having 

 red grains, did not all display quite the same degree of redness. The 

 palest reds generally gave 3 : i segregations in the next generation. 

 They had, so to speak, only one dose of red. The next palest might 

 breed true, but give 3 : i segregations in Fg after crossing with 

 white ; or they might give a 1 5 : i segregation immediately in the 

 next generation. They had two doses of red, either by being 

 homozygous for one of the genes or by being heterozygous for two 

 of them. It was dosage of red-producing allelomorphs that mattered, 

 not the particular genes involved. Thus genes with effects large 

 enough for them to be followed individually by the mendelian 

 method, could be shown to have similar effects, and effects which 

 were cumulative, that is to say supplementing one another, on the 

 phenotype. 



It was realized by Nilsson-Ehle and independently by East that 

 similar and supplementary action provided the basis for the graded 

 genetical levels demanded by continuous variation. And, if the effects 

 of the single gene differences were also small compared with the 

 non-heritable variation, as Johannsen's experiments had show^n to 

 be possible, no mendelian ratios would ever be obtained even though 

 the genes segregated in the mendelian fashion. Mendelian inheritance 

 was not limited to cases where it could be detected by the mendelian 

 method. It might indeed be regarded as covering the whole of 

 heritable variation. 



We can thus picture a spectrum of variation. At the one extreme 

 are gene differences of so large an expression in the phenotype that 

 they stand out both from their fellows and from the non-heritable 

 agencies which also cause variation in the character. These genes will, 



I InihiilsofGcueiics 65 E 



